Disclosed herein is a phase change magnetic ink and an in situ process for preparing a phase change magnetic ink.
Magnetic Ink Character Recognition (MICR) ink contains a magnetic pigment or a magnetic component in an amount sufficient to generate a magnetic signal strong enough to be readable via a MICR reader. Generally, the ink is used to print all or a portion of a document, such as checks, bonds, security cards, etc.
U.S. Pat. No. 5,667,924, which is hereby incorporated by reference herein in its entirety, describes in the Abstract thereof an image character recognition process comprising forming an image in a predetermined size and predetermined shape on a receiver member with a marking composition containing a superparamagnetic component, detecting the predetermined size and predetermined shape of the formed image by placing the resulting image bearing receiver member in a magnetic sensing field comprising at least one magnetic sensor and forming a magnetic size and magnetic shape profile map of the detected image, and recognizing the detected image by comparing the magnetic size and magnetic shape profile map of the formed image with reference magnetic profile image maps.
MICR inks or toners are made by dispersing magnetic particles into an ink base. There are numerous challenges in developing a MICR ink jet ink. For example, most ink jet printers limit considerably the particle size of any particulate components of the ink, due to the very small size of the ink jet print head nozzle that expels the ink onto the substrate. The size of the ink jet head nozzle openings are generally on the order of about 40 to 50 microns, but can be less than 10 microns in diameter. This small nozzle size requires that the particulate matter contained in an ink jet ink composition must be of a small enough size to avoid nozzle clogging problems. Even when the particle size is small than the nozzle size, the particles can still agglomerate or cluster together to the extent that the size of the agglomerate exceeds the size of the nozzle opening, resulting in nozzle blockage. Additionally, particulate matter may be deposited in the nozzle during printing, thereby forming a crust that results in nozzle blockage and/or imperfect flow parameters.
Further, a MICR ink jet ink must be fluid at jetting temperature and not dry. An increase in pigment size can cause a corresponding increase in ink density thereby making it difficult to maintain the pigments in suspension or dispersion within a liquid ink composition.
MICR inks contain a magnetic material that provides the required magnetic properties. The magnetic material must retain a sufficient charge so that the printed characters retain their readable characteristic and are easily detected by the detection device or reader. The magnetic charge retained by a magnetic material is known as “remanence.” The “coercive force” of a magnetic material refers to the magnetic field H which must be applied to a magnetic material in a symmetrical, cyclically magnetized fashion to make the magnetic induction B vanish. The coercivity of a magnetic material is thus the coercive force of the material in a hysteresis loop whose maximum induction approximates the saturation induction. The observed remanent magnetization and the observed coercivity of a magnetic material depend on the magnetic material having some anisotropy to provide a preferred orientation for the magnetic moment in the crystal. Four major anisotropy forces determine the particle coercive force: magnetocrystalline anisotropy, strain anisotropy, exchange anisotropy, and shape anisotropy. The two dominant anisotropies are: 1) shape anisotropy, where the preferred magnetic orientation is along the axis of the magnetic crystal, and 2) magnetocrystalline anisotropy, where the electron spin-orbit coupling aligns the magnetic moment with a preferred crystalline axis.
The magnetic material must exhibit sufficient remanence once exposed to a source of magnetization in order to generate a MICR-readable signal and have the capability to retain the same over time. Generally, an acceptable level of charge, as set by industry standards, is between 50 and 200 Signal Level Units, with 100 being the nominal value, which is defined from a standard developed by ANSI (American National Standards Institute). A lesser signal may not be detected by the MICR reading device, and a greater signal may not give an accurate reading. Because the documents being read employ the MICR printed characters as a means of authenticating or validating the presented documents, it is important that the MICR characters or other indicia be accurately read without skipping or misreading characters. Therefore, for purposes of MICR, remanence is preferably a minimum of 20 emu/g (electromagnetic unit/gram). A higher remanence value corresponds to a stronger readable signal.
Remanence tends to increase as a function of particle size and the density of the magnetic pigment coating. Accordingly, when the magnetic particle size decreases, the magnetic particles to experience a corresponding reduction in remanence. Achieving sufficient signal strength thus becomes increasingly difficult as the magnetic particle size diminishes and the practical limits on percent content of magnetic particles in the ink composition are reached. A higher remanence value will require less total percent magnetic particles in the ink formula, improve suspension properties, and reduce the likelihood of settling as compared to an ink formula with higher percent magnetic particle content.
Additionally, MICR ink jet inks must exhibit low viscosity, typically on the order of less than 15 centipoise (cP) or about 2 to 8 cP at jetting temperature (jetting temperature ranging from about 25° C. to about 140° C.) in order to function properly in both drop-on-demand type printing equipment, such as thermal bubble jet printers and piezoelectric printers, and continuous type printing apparatus. The use of low viscosity fluids, however, adds to the challenge of successfully incorporating magnetic particles into an ink dispersion because particle settling will increase in a less viscous fluid as compared to a more viscous fluid.
Magnetite (iron oxide, Fe2O3) is a common magnetic material used in MICR ink jet inks. Magnetite has a low magnetocrystalline anisotropy, K1, of −1.1×104 J/m3. An acicular crystal shaped magnetite, in which one crystal dimension is much larger than the other, has an aspect ratio of the major to minor size axis of the single crystal (Dmajor/Dminor) of 2.1 or larger, helps to augment the magnetic remanence and coercivity performance in inks. Acicular magnetite is typically 0.6 X 0.1 micron in size along the minor and major axis, respectively, and has a large shape anisotropy (6/1). Typical loading of iron oxide in inks is about 2 to 40 weight percents. However, due to the larger sizes and aspect ratio of acicular crystal shaped magnetite particles, they are difficult to disperse and stabilize into inks, especially for use in ink jet printing. Moreover, spherical or cubic magnetites are smaller in size (less than 200 nanometers in all dimensions), but have low shape anisotropy (Dmajor/Dminor) on order of about 1 . Consequently, because of the low overall anisotropy, spherical or cubic magnetite has lower magnetic remanence and coercivity, and loadings higher than 40 weight percent are often needed to provide magnetic performance. Thus, while spherical and cubic magnetite have the desired smaller particle size of less than 200 nanometers in all dimensions, the much higher loading requirement also makes them very difficult to disperse and maintain a stable dispersion. Moreover, such high loadings of the inert, non-melting magnetic material can interfere with other ink properties, such as adhesion to the substrate and scratch resistance. Consequently, this lessens the suitability of magnetites for ink jet printing inks.
Additionally, because magnetite has a specific gravity of approximately 7, magnetite has a natural tendency to settle to the bottom of a fluid ink composition. This can result in a non-homogeneous fluid having an iron oxide-rich lower layer and an iron oxide-deficient upper layer. Moreover, suitable ink jet oxides must generally be hydrophilic in nature in order to provide good dispersion characteristics and to provide good emulsion properties. The latter parameters related directly to the ability of the magnetic particle to exhibit minimum settling and to further demonstrate the proper wetting of the magnetic particle with the other water-soluble ingredients generally present in an ink jet ink composition.
The problems associated with using iron oxide in MICR ink jet inks have been addressed in several ways. For example, an MICR ink has been prepared using a combination of surfactants in conjunction with a very small particle size metal oxide component aimed at maintaining a useful suspension or dispersion of the magnetic component within the ink composition. Another means of achieving an ink jet compatible MICR-readable printing ink is to coat the metal magnetic material with a specific hydrophilic coating to help retain the particulate magnetic metal in suspension.
Another type of ink used for MICR ink jet printing is iron complex pigment ink, such as xFerrone™ based Versalnk™. These inks are compatible with certain commercially available printers and have a variety of uses, such as ensuring reliable scanning of checks and eliminating delays at store checkout lines.
U.S. Patent Publication Number 2009/0321676A1, which is hereby incorporated by reference herein in its entirety, describes in the Abstract thereof an ink including stabilized magnetic single-crystal nanoparticles, wherein the value of the magnetic anisotropy of the magnetic nanoparticles is greater than or equal to 2×104 J/m3. The magnetic nanoparticle may be a ferromagnetic nanoparticle, such as FePt. The ink includes a magnetic material that minimizes the size of the particle, resulting in excellent magnetic pigment dispersion stability, particularly in non-aqueous ink jet inks. The smaller sized magnetic particles of the ink also maintain excellent magnetic properties, thereby reducing the amount of magnetic particle loading required in the ink.
Water-based MICR ink is commercially available. Water-based MICR ink requires special print-heads to be used with certain ink jet printing technology such as phase change or solid ink technology. There is further a concern with respect to possible incompatibility when operating both solid ink and water-based ink in the same printer. Issues such as water evaporation due to the proximity to the solid ink heated ink tanks, rust, and high humidity sensitivity of the solid ink are issues which must be addressed for implementation of a water-based MICR ink in a solid ink apparatus.
Currently, there are no commercially available phase change or solid MICR inks. There is a need for a MICR ink suitable for use in phase change or solid ink jet printing. There are numerous challenges in developing a MICR ink suitable for use in phase change or solid ink jet printing. MICR phase change ink processes are particularly challenging with magnetic pigments because (1) inorganic magnetic particles are incompatible with the organic base components of phase change ink carriers, and (2) magnetic pigments are much denser than typical organic pigments (the density of iron is about 8 g/cm3, for example) which can result in unfavorable particle settling, and (3) uncoated metal magnetic nanoparticles are pyrophoric thus presenting a safety issue. Further, in order to make a solid ink composition from these particles, a complex process would be required including preparation of the magnetic particles, washing of the nano-particles, followed by a multi-step procedure for fabrication of solid ink compositions containing the magnetic pigments.
Currently available MICR inks and methods for preparing MICR inks are suitable for their intended purposes. However, a need remains for MICR ink jet inks that have reduced magnetic material particle size, improved magnetic pigment dispersion and dispersion stability along with the ability to maintain excellent magnetic properties at a reduced particle loading. Further, a need remains for MICR phase change inks that are suitable for use in phase change ink jet printing technology. Further, a need remains for a process for preparing a MICR process that is simplified, environmentally safe, capable of producing a highly dispersible magnetic ink having stable particle dispersion, allowing for safe processing of metal nanoparticles, cost effective, and green.
The appropriate components and process aspects of the each of the foregoing U. S. Patents and Patent Publications may be selected for the present disclosure in embodiments thereof. Further, throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.